Field of the Invention
The present invention relates to a process for preparation of a wholly carbonaceous diaphragm for acoustic equipment use. More particularly this invention relates to a process for preparation of a wholly carbonaceous diaphragm which is suited for the age of digital audio from the various viewpoints such that as compared with the conventional diaphragms it is surpassingly light in weight and highly elastic as the diaphragm for speaker, headphone or microphone use; and because of the large velocity of the propagation of sound and moreover the excellent rigidity its deformation due to external forces is very little, with the distortion of sound being small and the sound range being broad, so that it can emit a sound of clear tone quality.
Prior Art
As the conventional diaphragms use has been made of those which are obtained by compounding glass fibers or carbon fibers with paper (pulp), plastic, or their raw materials, as the basic material, or those which are obtained by processing metals such as aluminum, titanium, magnesium, beryllium, boron, etc., or alloys thereof, or the raw materials such as nitrides, carbides, borides, etc. However, paper, plastic, and the compounded raw materials thereof are small in the ratio of Young's modulus to density, and hence the velocity of sound is slow causing split vibration at a specific mode, and the frequency characteristics are markedly lowered especially in the high frequency wave region, so that it is difficult to obtain clear tone quality.
In addition to the above, there are defects such that it is not only liable to be influenced by the external circumstances such as temperature, humidity, etc. but also susceptible to qualitative degradation and fatigue on standing of the material, which cause the lowering of the characteristics. On the other hand, when in use of metallic disks of aluminum, titanium, magnesium, etc., although the velocity of sound is fast showing excellent properties as compared with paper, plastic, etc., the velocity of sound is not as yet sufficiently fast, and the internal loss is also small, so that there are defects such that a sharp resonance phenomenon takes place in the high frequency wave region, or fatigue on standing such as creep, or the like, of the material is brought about to lower the characteristics. Although beryllium and boron are ideal materials having excellent physical constants, they are highly expensive because of scantiness in natural resource, and industrially their processing is considerably difficult.
Besides the above, by paying attneiton to carbon materials, there has been developed a wholly carbonaceous diaphragm for acoustic equipment use.
Among the wholly carbonaceous diaphragms heretofore obtained, those which are obtained without orienting to a high degree the highly elastic material that is the important element for exhibiting a high velocity of sound are found to be difficult to realize the high velocity of sound. Also, those which are obtained by molding only a high elastic material blended with a binder not only require a lot of time in the treatment for making the carbon precursor but also are liable to cause deformation or distortion of the baked product. On the other hand, the process for preparation of the carbon diaphragms by CVD, or the like, is not suited for mass production, and moreover the manufacturing step is complicated and a high velocity of sound can hardly be attained.
The object of this invention is to provide a process for preparation of a wholly carbonaceous diaphragm for acoustic equipment use, which is free from these problems.
The present inventors, in view of the above-described defects heretofore encountered, made an extensive investigation in order to develop a process for preparation which can produce such a wholly carbonaceous diaphragm capable of exhibiting to the highest degree the functional characteristics of carbon materials, while avoiding complicated steps in the manufacture and especially without injuring the shape of the product when a suitable form is to be given thereto, and as the result the invention of the present application could be achieved.
In the invention of this application, paying special attention to the fact that the crystals of graphite having a high crystallinity are tabular and flat, and its theoretical elastic modulus is 1020 GPa showing an extremely high value as compared with other materials, one creative method was devised in that after a compounded carbon thin film has been prepared by orienting to a high degree the crystals of graphite having a high crystallinity along the direction of the film surface using a resin charcoal as a binder, said carbon thin film is given an optional form. On the other hand, during the research on the material of a binder such that when the carbon material has been given a form, it does not cause deformation, maintaining the form initially given even after baking, it was found out that a product obtained as a blend polymer of a chlorine-containing resin and a thermosetting resin containing a triazine ring can show the behavior of a thermoplastic resin as such when given a form, rendering it possible to be readily processed by thermoplastic molding, whereas as soon as the solvent or plasticizer contained therein is thereafter removed by heating in the air, cross linking reaction and curing reaction take place between the chlorinecontaining resin and the triazine, and then the reaction product becomes insoluble and infusible even by heat and solvent. Moreover, it was also found out that the material for molding thus obtained by compounding the powder of graphite by the use of such a carbonizing binder not only can maintain the form initially given with high precision even after baking but also can exhibit to the highest degree the functional characteristics of carbon materials.